220 



SENSE OF SIGHT. 



Fig. 94. 



Posterior Segment of Transverse Sec- 

 tion of the Globe of the Eye seen 

 from within. 



1. Divided edge of three tunics. The 

 membrane covering the whole internal 

 surface is the retina. 2. Entrance of 

 optic nerve with arteria centralis retina 

 piercing its centre. 3, 3. Ramifications 

 of arteria centralis. 4. Foramen of 

 Sommering, in centre of axis of eye ; 

 the shade from sides of the section ob- 

 scures the limbus luteus which sur- 

 rounds it. 5. A fold of the retina, which 

 generally obscures the foramen of Som- 

 mering after the eye has been opened. 



posed of several thin laminae in superposi- 

 tion, which have been compared to horn ; 

 and hence the name of the membrane : 

 but Mr. T. Wharton Jones 1 denies this,' and 

 describes it as consisting merely of inter- 

 weaving bundles of fibres. Like corneous 

 tissue in general, it possesses neither blood- 

 vessels nor nerves. In animals, the dens- 

 ity and convexity of the cornea vary with 

 the media in which they live, and with 

 the condition of the other refractive parts 

 of the eye. In old age, the membrane 

 is harder, tougher, and less transparent 

 than in youth ; and it frequently becomes 

 completely opaque in its circumference, 

 presenting the appearance called arcus 

 senilis, in German, Crreisenbogen. 

 Nerves have been traced into the sub- 

 stance of the cornea. They are ramifica- 

 tions of the ciliary. 2 



The aqueous humour is a slightly viscid 

 fluid, which occupies the whole of the space 

 between the posterior surface of the cornea 



Fig. 95. 



Vertical Section of the Sclerotic and Cornea, showing the continuity of their tissue between the 



dotted lines. 



a. Cornea, b. Sclerotic. In the cornea, the tubular spaces are seen cut through, and in the sclero- 

 tic, the irregular areolse. Cell-nuclei, as at c, are seen scattered throughout, rendered more plain by 

 acetic acid. Magnified 320 diameters. 



and the anterior surface of the crystalline. This space is divided by 

 the iris into two chambers an anterior and a posterior the latter 

 being the small interval between the hinder surface of the iris, and 

 the anterior surface of the crystalline. Sir David Brewster 3 erro- 

 neously asserts that the posterior chamber contains the crystalline and 



1 Introduction toW. Mackenzie's Practical Treatise on Diseases of the Eye, Lond., 1840. 



2 Lond. Med. Gaz., Oct., 1845, cited from Miiller's Archiv. 



3 A Treatise on Optics, edit, cit., p. 241. 



